Dustwood is technically not a wood from a specific tree. It is an created fuel for fires.
Starting with the sawdust from a sawmill, adding a mixture of wax or pitch, and a few other things, the mixture is pounded or pressed toghether tightly. It is normally smashed into a round container, so when it is slipped out, it looks like a small perfectly round "log". (actually a lathed log, but you get the idea). It can either be used as a log or slices of a log can be used for a fire.
Dustwoods can contain extras. The most popular are fragrences (flowering or pine) or colorful fire (extra metal bits add color to the fire). Certain dustwoods are made as kindling. They have pitch and some other flamable materials so they will catch quick and easy.
Many Rangers would use this as they are quite handy and store well.